1. Field of the Invention
This invention relates generally to cargo carrying trailers and more particularly to stable, true tracking trailers that can be cross-coupled into a train of trailers.
2. Description of the Prior Art
Trailers are commonly used to transport cargo from one place to another. These trailers are usually hitched to some means of locomotion, such as a tugger or a forklift, and towed. To be tractable, trailers must have at least one fixed wheel in contact with the ground at all times. Trailers supported fully by castor wheels have the tendency to meander, and are generally unmanageable. Trailers found in the prior art show indications that their designers have gone to great lengths to assure that at least one fixed wheel remains on the ground. For instance, in U.S. Pat. No. 2,649,965, J. P. King et al discloses a trailer that keeps its fixed wheels in contact with the ground by raising the castors from the plane of contact defined by the fixed wheels. While effective in keeping a fixed wheel on the ground, this approach introduces a pronounced "tippiness" or rocking motion to the trailer.
A. March teaches another method for keeping a fixed wheel on the ground in U.S. Pat. No. 3,294,417. In March's patent a fixed wheel is connected to a center point of a lever which has an end pivotally attached to the bottom of a trailer and the other end weighted, the weight urging the wheel into ground contact at all times.
A problem that the prior art does not address is how to design a stable trailer that has one fixed wheel in contact with the ground at all times without resorting to exotic constructions and without introducing a rocking motion to the trailer.
Often times, one trailer does not have sufficient cargo carrying capability to accomodate a complete load, necessitating the coupling of a number of trailers into a trailer train. The lead trailer of the train is then hitched to a means of locomotion and the entire train is towed. A problem encountered when towing such a train is that the trailers do not exactly follow the path of the locomotive means. In direct coupled trailers, where the beds of the trailer are hitched together, there is a pronounced tendency for the trailer train to cut corners. Contrasting, where the pivoting front axle of the trailor is attached to the bed of the trailer in front of it, the train is predisposed to exaggerate the corner that the locomotive means follows. Both corner cutting and exaggeration can make tight turns in crowded environments, such as warehouses, very difficult to accomplish.
To overcome these problems, a number of true tracking trailers have been designed. For instance, one type of true tracking trailer adapted to be towed in a train has wheel steering linkages connecting the trailer to the one in front of it. Another type of true tracking trailer uses a cross link coupling system to steer the trailers.
Other examples of true tracking trailers found in the prior art include the invention disclosed by U.S. Pat. Nos. 776,405 and 2,078,756. U.S. Pat. No. 2,078,756, for instance, discloses a vehicle having four independently steerable wheels that are each connected by linkages, pivots, swivels and other coupling members to the trailers ahead. A problem with true tracking trailers of this type is that the linkages, pivots, swivels and other associated members add to the complexity of the vehicle. This, in turn, increases the cost and probability of failure of the final product. A second problem with true tracking trailers having pivotal wheels connected by linkages is that the bed of the trailer must be relatively high off the ground so that the wheel steering linkages under the bed have adequate ground clearance. This leads to top-heavy trailers and increases loading difficulties.
An example of true tracking trailers which do not use wheel linkages is found in U.S. Pat. No. 2,313,235 of C. C. Grove which discloses cross-link coupling means for towing a train of trailers. In Grove's patent first and second links couple opposite corners of two trailers, the links each being provided with a longitudinal slot and being connected together by a floating pivot retained in the slots.
A problem with connecting links together with a floating pivot is that the turning angle of the trailer is limited by the pivot which engages the end of the slots.
A problem that cross-coupled trailers of the prior art do not address is what the dimensional relationship between the cross link lengths and the trailer body's dimensions should be in order to insure that the trailers are absolutely true tracking.